The HTML::Tree suite seems to have some XML capabilities. HTML::Element has an XML dump method: $h->as_XML(), which might be a first step, depending on what you want to do.

There is also a HTML::DOMbo module, which turns your HTML tree into an XML tree, and AFAICS, lets you use all of the DOM tools you want on it.

While I have been using HTML::Tree a lot recently (and I highly recommend it for doing most anything with HTML), I haven't experimented with the XML stuff yet. But it seems promising.